1. Field of the Invention
The present invention relates to a photo-writing type recording medium usable as a so-called electronic paper is written an image with light, can retain the image thus written, and is deleted the image as necessary so that another image can be written therein and a manufacturing method therefor.
2. Description of the Related Art
In recent years, a photo-writing type space modulation device having a photo-conductive switching element and a display element in combination has been developed and put to practical use in projector, etc. as a lightbulb, and its applicability to the art of optical data processing has been studied as described in “Liquid crystal space modulator and data processing”, Liquid crystal, vol. 2, No. 1, 1998, pp. 3–18.
While a predetermined voltage is being applied to the display element, the photo-writing type space modulation device changes the impedance of the photo-conductive switching element according to an amount of light received and controls the voltage applied to the display element to drive the display element so that an image is displayed thereon. In particular, a separable photo-writing type recording medium, which is a photo-writing type space modulation device employing an a display controlling element having memory properties, has attracted attention as an electronic paper medium.
As display controlling elements (display elements) for photo-writing type recording medium there have been studied liquid crystal display elements such as nematic liquid crystal which has been dispersed in a polymer to have memory properties, cholesteric liquid crystal and ferroelectric liquid crystal, electrophoretic element, electric field rotation element, toner electric field transfer element, materials obtained by encapsulating these elements, etc.
As optical switching elements capable of controlling voltage or current according to the amount of light received, there have been studied, e.g., amorphous silicon element for use in the art of electrophotography, OPC element having a function-separating type two-layers structure having an organic photo-conductor, etc. Further, the inventors have studied an OPC element comprising a charge generation layer (CGL) formed on the upper and lower sides of a charge transport layer (CTL) (hereinafter referred to as “dual CGL structure”). In particular, an OPC element requires no high temperature heat treatment and thus is the OPC element advantageous in that it can be applied to a flexible substrate such as PET film. Further, the OPC element requires no vacuum process and thus is advantages in that the OPC element can be manufactured at a low cost.
Among the foregoing structures, the dual CGL structure can be driven by an a.c. voltage. Even when a liquid crystal element is used as a display element, the bias component contained in the voltage applied prevents the displayed image from being burn-in due to the transfer of ion. Thus, the dual CGL structure is a very useful structure. The carrier to be used for driving may be either positive or negative.
FIG. 1 illustrates a schematic sectional view of an optical switching element having the dual CGL structure. FIG. 1 shows the state of carrier and electron generated when the optical switching element is irradiated with light. The optical switching element of FIG. 1 has an upper charge generation layer 10, a charge transport layer 12, a lower charge generation layer 14, a transparent electrode layer 18, and an electrode layer 16 sequentially laminated on the surface of a transparent substrate 19.
When the optical switching element is irradiated with light, carrier c and free electron e are generated in the upper charge generation layer 10 and the lower charge generation layer 14. If this occurs while the optical switching element is under the application of an electric field such that the electrode layer 16 is a positive electrode and the transparent electrode layer 18 is a positive electrode, carrier c generated in the upper charge generation layer 10 is injected into the charge transport layer 12 while free electron e rushes into the transparent electrode layer 18. The carrier c which has been transported combines with the free electron e generated in the lower charge generation layer 14 while the carrier c generated in the lower charge generation layer 14 is injected into the electrode layer 16. As a result, an electric current flows. When the electric field is inverted, a direction of electric current is also inverted. Accordingly, the optical switching element having such a structure can be driven by the application of an a.c. voltage.
FIG. 2 is a conceptual diagram of a photo-writing type recording medium in which an optical switching element having such a dual CGL structure is applied to an electronic paper. While FIG. 2 shows a photo-writing type recording medium having a d.c. component-removing film, such a d.c. component-removing film is not an essential constituent. The photo-writing type recording medium in FIG. 2 has integrally an optical switching element having a transparent substrate 19, a transparent electrode layer 18a, a lower charge generation layer 14, a charge transport layer 12 and an upper charge generation layer 10; a d.c. component-removing film provided as a functional layer 20 on the optical switching element; and a liquid crystal display element having a spacer 24, a liquid crystal 22, a transparent electrode layer 18b and a transparent substrate 19 provided on the functional layer 20. In operation, an a.c. voltage is applied between the transparent electrode layers 18a and 18b. In general, light represented by an arrow performs photo-writing.
By electrically connecting the foregoing photo-writing type recording medium having integrally the optical switching element and a functional element to a driving mechanism for driving such a photo-writing type recording medium, a device having various functions can be produced. Further, the driving mechanism is structured to be separable from the photo-writing type recording medium, the photo-writing type recording medium can be separated from the main body of the device so that the photo-writing type recording medium can be subjected to circulation or distribution.
However, the dual CGL structure is disadvantageous in that the dual CGL structure has an insufficient photosensitivity. For example, in the case where as a charge generation material there is used perylene, the dual CGL structure needs a light amount as great as several milliwatts per cm2 to make positive recording (irradiated area shows a high reflectance while unirradiated area shows a low reflectance) for forming a monochromatic image as described in JP-A-2000-180888.
The term “photosensitivity” as used herein means that when the optical switching element is irradiated with a predetermined amount of light or when the optical switching element is applied a predetermined voltage, the optical switching element shows a reduced resisitivity component in the impedance thereof. The predetermined volume of light is normally in a range of from about 50 μW/cm2 to about 500 μW/cm2, or about 1 mW/cm2 when the optical switching element is irradiated large amount of light. The predetermined voltage is normally in a range of from about 50 VOP to about 500 VOP. An OPC element exhibits an impedance resistivity as very high as in a range of from several hundreds of megaohm per cm2 to several gigaohm per cm2 in darkness but exhibits an impedance resistivity in a range of from hundreds of kiloohm per cm2 to scores of megaohmper cm2. On the other hand, a display element normally exhibits an impedance resistivity of from several megaohm per cm2 to about 100 MΩ per cm2. Therefore, as the resistive component decreases more upon the irradiation with light, this decreasing makes greater contribution to enhancement of recording sensitivity or expansion of recording margin.
Therefore, an aim of the invention is to solve the problems of the foregoing photo-writing type recording medium having a dual CGL structure in the related art. In other words, an aim of the invention is to provide a photo-writing type recording medium with a dual CGL structure having a high photosensitivity. Another aim of the invention is to provide a manufacturing method for manufacturing a photo-writing type recording medium with a dual CGL structure having a high photosensitivity.
In the art of ordinary electrophotography, 1) efficiency of generation of carrier and 2) efficiency of injection of carrier is essential for the enhancement of sensitivity to light. The dual CGL structure must meet further requirement for 3) efficiency of release of carrier to a charge transport layer. In other words, the dual CGL structure is arranged such that carrier generated in the charge generation layer rushes into the charge transport layer from which the carrier is then released into the charge generation layer. Accordingly, the dual CGL structure must be arranged such that carrier can be easily injected from the charge generation layer into the charge transport layer as well as can be easily released from the charge transport layer to the charge generation layer.
The foregoing performances are requirements peculiar to the dual CGL structure and is different from required performances of the function-separating type two-layers structure in the electrophotographic photoreceptor. In the case of selection of charge generation layer and charge transport layer for the function-separating type two-layers structure in the electrophotographic photoreceptor, it is effective to design the ionization potential of the charge generation material greater as much as possible than that of the charge transport material so that charge generated in the charge generation layer can be efficiently injected into the charge transport layer, i.e., the efficiency of injection of charge can be enhanced.
In the case that a CTL (carrier transport layer) is hole transfer type, since hole moves on HOMO in CGL and CTL, a barrier layer is formed when |HOMOcgl|−|HOMOctl| is smaller than 0 from the standpoint of energy level as shown in FIG. 3. Therefore, it is necessary that |HOMOcgl|−|HOMOctl| be greater than 0. Accordingly, in order to give a higher efficiency of injection, it is an important design guidance that this difference is positive and great. As shown in FIG. 3, this energy level corresponds to ionization potential. Further, since this ionization potential can be measured, materials can be selected and designed in view of ionization potential.
On the other hand, the dual CGL structure is arranged such that charge which has been injected from the charge generation layer into the charge transport layer is then released to the other charge generation layer. Further, in the case where the dual CGL structure is driven by an a.c. voltage, the release of carrier from the charge generation layer to the charge transport layer and the re-release of carrier from the charge transport layer to the charge generation layer occur in the same charge generation layer. Therefore, in the case where |HOMOcgl|>>|HOMOctl| (the symbol “>>” means that the former is extremely greater than the latter), no barrier layer is formed when carrier rushes into the charge transport layer but a barrier layer is formed when carrier then rushes into the charge generation layer. Accordingly, in the dual CGL structure, it is difficult to enhance transport efficiency. Therefore, the dual CGL structure is disadvantageous in that it cannot exhibit a sufficiently lowered resisitivity when irradiated with light. As a result, it is required that large amount of light is irradiated or the recording margin is reduced. This is a problem.